Solar Photo-Voltaic (PV) cells have been used to convert solar light energy into electrical energy for many years. Such cells are used to power many kinds of appliances from small calculators, through solar powered vehicles, and up to large solar cells arrays capable of producing large amounts of power for the purpose of supplying regional power grids.
FIG. 1 of the prior art is an exemplary arrangement of a solar cell array 1000, which is comprised of a multitude of solar cell panels 100 arranged in a grid formation. Each solar cells panel 100 is comprised of a grid of individual solar cells 10 (as seen in FIG. 2a), which are normally connected in series and/or in parallel.
As shown in the present illustration, certain areas of the solar cell array 1000 are shaded, for example shading from solar cells panel 100s and shading from building 200s. Just as a nearby building 200 can cast shade, other objects such as nearby trees or clouds passing in the sky can cast shade. The state of the shade, its extent, its location upon the solar cell array 1000 changes with time, and with environmental conditions. Clearly, any reduction in the light radiation upon the solar cell array 1000 diminishes its capacity for production of electrical output.
FIG. 2a of the prior art is an isometric schematic illustration of an illustrative, only as a possible example exemplary embodiment of a solar cell 10. There are also other types of PV solar cells than the type depicted here, e.g., organic cells. The solar cell 10, of Mono Crystalline Silicon type, is made from two types of semiconductors: an N-type semiconductor 11 and a P-type semiconductor 12. Since the N-type semiconductor 11 has an excess amount of free electrons 13 and the P-type semiconductor 12 has an excess of free holes 14, a depletion region is created when the two semiconductor materials are connected and that forms a diode. When a semiconductor is exposed to light, the semiconductor releases electrons to move freely within the semiconductor. When a diode is exposed to light, the release of electrons causes the electrical field within the solar cell to increase due to the formation of electron-hole pairs within the solar cells, causing a potential difference (i.e., voltage) to appear between the front contact 16 and the back contact 15. Adding a load between the front contacts 16 to the back contact 15 would cause a current to flow between the contacts. The anti-reflective coating 17 is used to reduce the loss of photons that may be reflected off of the semiconductor material thereby reducing its efficiency. A glass cover 18 is used to protect the solar cell from outside elements and contaminates.
FIG. 2b of the prior art describes a well-known electrical model for a photo-voltaic solar cell known as the “ten-parameter” model.
One of the major problems occurring within large solar cell arrays is the presence of weak cells. Weak cells supply less power than other cells in the array and change from supplying power to the external load to becoming a load themselves that will suck up electrical power from the other solar cells. The weak solar cells could appear due to degradation in the performance of the cells due to aging as well as shade falling on the cells. Such shade may be cast by nearby objects such as buildings and trees located near the array or other solar cells panels within the array, by clouds, or by dust present on the solar cell surface.
FIG. 3a of the prior art is a schematic electrical representation of a solar cell equipped with a bypass diode.
The present figure depicts an exemplary solution, used to address the issue (as described with regard to the previous illustration), by adding a bypass diode D in parallel to a solar cell 10. When such a cells' performance degrades due to aging or shading, the internal current it produces would be lower than the internal currents produced by the other cells in the array. The bypass diode would permit the pass of the whole current not only through the bypassed weak solar cell but also through the bypassing diode itself.
FIG. 3b of the prior art is a schematic electrical representation of a solar cells chains block 60 including a set of solar cells chains 50, each equipped with a blocking diode D.
An option for avoiding the use of weak cells is by using blocking diodes. Some solar cells chains blocks 60 are comprised of several solar cells chains 50 of series of solar cells connected in parallel. For each solar cells chain 50, there is one blocking diode. When the performance of one or more of the solar cells 10 in a specific solar cells chain 50 performance weakens, e.g., due to temperature differences, the voltage produced by the solar cells chain 50 drops. If there is no blocking diode, current flows from other solar cells chains 50 into the weak solar cells chain 50, thereby reducing the total current flowing out of the solar cells chains block 60. With the blocking diode, no current would flow back into the weak solar cells chain 50.
However, using blocking diodes disables a whole solar cell chain 50, which may include fully functional solar cells 10. This reduces the output power of the solar cells chains block 60 by the number of cells in the blocked chain.
Blocking diodes 30 are mandatory whenever the solar cells chains block 60 needs to be protected from backward current from the grid (e.g. during night where no safety switches installed). If it is an independent system with a charge controller, blocking diodes 30 are redundant. In such a case, only bypass diodes are used.
FIG. 3c of the prior art is a schematic electrical representation of a solar cells chain 50 of four solar cells 10, each equipped with a bypass diode D.
The present figure illustrates an exemplary arrangement of four solar cells 10 serially connected into a solar cells chain 50 with a bypass diode D connected in parallel to each of the solar cells. In this figure, one of the cells (104) is shaded and receives only two thirds of the illumination received by the other cells.
FIG. 3d is a graph of simulation results of the effect of different bypass diodes on the performance of a solar cells chain 50, according to the present invention.
The present figure shows the simulations result graphs of the solar cells chains' 50 performance using different types of bypass diodes with ranging forward voltage (Vf). The continuous line depicts the case where no bypass diode is used. The dashed lines depict the effect that the different bypass diodes have. From the graph, it is easy to see that as the diodes' Vf increases, the efficiency of the chain decreases. However, the maximum power point (MPP) does not change in this specific case. Since the bypass diode eliminates a cell, it is not possible to associate the weak cell to the power supplied by the solar array to the load. Using bypass diodes reduces the output power of the solar cells panel 100 as the number of solar cells 10 being used decreased causing the solar cells panel 100 to operate with less than the maximum number of solar cells 10.
FIG. 4a of the prior art is a schematic block diagram of a system for harvesting solar energy.
In order to achieve the optimum performance of a solar cell structure (a chain, a panel, an array etc.), a Maximum Power Point Tracker (MPPT) 102 may be used.
The MPPT 102 searches the IN curve of the solar cell structure for the Maximum Power Point (MPP) and presents an optimal electrical load to a solar cell structure and produces a voltage suitable for the load. The MPPT 102 is sometimes referred to as a “DC/DC converter”.
The solar cells 10 provide a voltage V1. The MPPT 102 sets its load for the solar cells 10 in order to enable the solar cell 10 to operate at its MPP and converts the solar cell's 10 output voltage V1 to a voltage V2 suitable for the DC/AC inverter 104. The DC/AC inverter 104 inverts the DC voltage V2 into AC voltage, which can be supplied to the power grid 106.
The main disadvantages of the MPPT 102 are its cost and it efficiency. The MPPT 102 is too expensive to be used by every solar cell 10 or every solar cells panel 100 and is usually used only at the whole solar cells array 1000 level. But, since different solar cells panels 100 may have different IN curves, i.e. different MPPs (due to manufacturing tolerance, partial shading, etc.). This architecture may cause some solar cells panel 100 to perform below their MPP, resulting in the loss of energy. The other main disadvantage of the MPPT 102 is its own efficiency. The best MPPTs 102 currently available have 97% efficiency. This means that at least 3% of the harvested power is wasted just by using an MPPT 102.
If the cells can be reconnected by a meandering scheme, the entire problem can be averted. The only caveat is that in order to perform meandering, the cells/panels have to be sorted by the maximal power that each can supply. Apparently the only way to discover this power is to use a sniffing algorithm which either uses a variable load by use of a DC-DC converter 102 or physically connects loads to test which perform better, but due to the possibility of the presence of local maxima on the PP curve, there is absolutely no assurance this would find the absolute maximum.
Another way to find the power of the cells/panels is by using the well-known “ten-parameters model” which is described in FIG. 2b and complies with the following formula:
  I  =            I      ph        -                  I                  0          1                    ⁡              (                              ⅇ                                                            (                                      V                    +                                          I                      ·                                              R                        s                                                                              )                                /                                  V                  T                                            ⁢                              n                1                                              -          1                )              -                  I                  0          2                    ⁡              (                              ⅇ                                                            (                                      V                    +                                          I                      ·                                              R                        s                                                                              )                                /                                  V                  T                                            ⁢                              n                2                                              -          1                )              -                  V        +                  I          ·                      R            s                                      R        sh              -          α      ⁢                        V          +                      I            ·                          R              s                                                R          sh                    ⁢                        (                      1            -                                          V                +                                  I                  ·                                      R                    s                                                                              V                br                                              )                          -          m                    
However, Iph, I01 and I02 depend on the temperature and Iph also depend on the illumination. But, this dependence can be used to acquire an estimation of the temperature and the illumination given a single current and voltage measurement when having the ten-parameters known in advance; however an initial temperature and wind speed measurement by a single thermometer and a single anemometer for the whole solar field is necessary to start the iterations: Once an initial cell temperature is known, new values for I01 and I02 can be substituted by the temperature dependency: Assume the parameters tested on cell temperature T0. Then, the parameters at another temperature T1 relate to the parameters at T0 by the following equations
                    I                  0          1                    ⁡              (                  T          1                )              =                                        I                          0              1                                ⁡                      (                          T              0                        )                          ·                              (                                          T                1                                            T                0                                      )                    3                    ⁢              ⅇ                              1.3            ·                          10              4                                ⁢                      (                                          1                                  T                  0                                            -                              1                                  T                  1                                                      )                                                  I                  0          2                    ⁡              (                  T          1                )              =                                        I                          0              2                                ⁡                      (                          T              0                        )                          ·                              (                                          T                1                                            T                0                                      )                                3            /            2                              ⁢              ⅇ                              0.65            ·                          10              4                                ⁢                      (                                          1                                  T                  0                                            -                              1                                  T                  1                                                      )                              andIPH≈Isc(T1)=Isc(T0)+α1(T1−T0)
where α1 is a constant that the cell manufacturer supplies, and is dependent on the cell geometry. If it is not known, it is possible to measure it easily in a short circuit experiment in different temperatures. Then, it is straightforward to go back to the model equation and substitute the new values received for Iph, I01, and I02. The model equation has to hold for the voltage and current measured, so a new value for Iph can be extracted from it easily by:
      I    ph    =      I    +                  I        01            ⁡              (                              ⅇ                                                            (                                      V                    +                                          I                      ·                                              R                        s                                                                              )                                /                                  V                  T                                            ⁢                              n                1                                              -          1                )              +                  I        02            ⁡              (                              ⅇ                                                            (                                      V                    +                                          I                      ·                                              R                        s                                                                              )                                /                                  V                  T                                            ⁢                              n                2                                              -          1                )              +                  V        +                  I          ·                      R            s                                      R        sh              +          α      ⁢                        V          +                      I            ·                          R              s                                                R          sh                    ⁢                        (                      1            -                                          V                +                                  I                  ·                                      R                    s                                                                              V                br                                              )                          -          m                    
This new value for Iph can be used to extract a new estimate on the illumination. This, in turn, affects a new estimate on the cell temperature (having a new value for the illumination and knowing the wind speed, means the cell temperature has to be modified.). Then, from the new temperature, newer values can be deduced for Iph, I01, and I02, which can be again introduced into the ten-parameters model equation to further modify the value of Iph. This iterative process can be either repeated a fixed number of times kept cycling until it converges, or as another option, an estimate of the open circuit voltage can be used, to know when to stop the iterations. Simulations run using Matlab software runs showed consistent convergence; however it is possible that in some cases it would not converge—so a possible way to exit the calculations would be to run a small number of iterations and accept the last value to stop the calculations when the difference between two successive values would be under a threshold, or to use an estimate of the open circuit voltage and check every time if the simulation reached it or to use any other of many possible limiting boundary techniques can be used.
There is therefore a need for a system and a method for improving the efficiency of solar cells, which will enable online assessment of solar cells MPP and perform online “partial meandering” as well as provide solutions to the issues stated above.